Dielectric lens for an antenna and manufacturing process thereof

Abstract: A dielectric lens which consists of a radome (1) which is a dome shell, a lid (5), and a foamy body (10) produced by fusion-molding of preexpanded beads. The foamy body is mounted in the radome, and the radome is closed with the lid.

Description: [Technical Field] The present invention relates to a dielectric lens, and more particularly to a dielectric lens used as an element of an antenna for receiving microwave for communication and broadcasting. [Background Art] A dielectric lens used as an element of an antenna for receiving microwave of 5GHz or more is conventionally produced by: mixing a resin, for example, polypropylene, polyethylene, polystyrene or the like, with ceramic powder, which acts as a foaming agent and as a dielectric constant conditioner; and molding the mixture into a dome. In the molding, the surface of the dielectric lens is solidified, and a radome layer is formed. The radome layer protects the inner foamy body from weathering and reinforces the foamy body. In the conventional manufacturing process, the dielectric lens is molded to be thick and accordingly to be heavy. For example, if a mixture of polypropylene and ceramic powder is molded into a dome which is about 180mm in diameter and about 76mm in height, the weight will be about 1kg. Thus, a large quantity of resin is necessary, and the cost of the dielectric lens is high. The thickly molded dielectric lens is likely to have a defect such as a sink mark and a swirl mark on the surface (radome layer) and a void inside. The defect can be fairly avoided by adopting injection compression molding. However, the injection compression molding requires a mold of a complicated structure and expensive facilities. Even in the injection compression molding, it is difficult to completely prevent occurrence of a sink mark, a swirl mark and a void. Further, a product by the injection compression molding has residual stress, which is a cause of deflection. WO 93/10572 relates to a dielectric material for antennas which is composed of individual pieces which appear essentially homogenous to signals whose frequency is such that their wavelength is greater than the dimension of the pieces. The dielectric material comprises some dielectrical pieces which collectively form a dielectic block which itself has the shape of a dielectric antenna to be produced. The pieces may be fixed to each other, for example by glueing, or may just lay near or on each other. A microwave length antenna comprises a radome cover which may also serve as a matching layer to reduce the amount of reflected signal at the lens surface. The radome cover is made of plastic and may be used as a vessel to contain the unbound plastic pieces. WO 89/08932 relates to a solid dielectric lens aerial comprising a lens of dielectric material transparent to radiation. The material for the solid dielectric lens aerial may be silica glass beads or silica flour which is bound together. The lens may comprise a shell formed of two halves and filled with the powder. It is the object of the present invention to provide a method of producing a light and inexpensive dielectric lens which does not have a defect such as a sink mark, a swirl mark and a void, and which does not require expensive facilities. This object is achieved by a method according to claim 1. According to the present invention which is defined in claim 1, the radome and the foamy body are molded separately, and the produced lens is less likely to have a sink mark, a swirl mark and/or a void compared with a conventional dielectric lens produced by integral molding. Also, since preexpanded beads are used for the foamy body, only a small quantity of resin is necessary, thereby lessening the weight. The radome is formed by ordinary thinwall injection molding, and the foamy body is formed by ordinary foaming. Therefore, such expensive facilities as to be used for injection compression molding are not required, and the cost for facilities is low. [Brief Description of the Drawings] These and other objects and features of the present invention will be apparent from the following description in connection with the accompanying drawings, in which: Fig. 1 is an explosive perspective view of a dielectric lens which is an embodiment of the present invention; and Fig. 2 is a sectional view of the assembled dielectric lens. [Best Mode for Carrying out the Invention] A dielectric lens according to the present invention and a manufacturing process thereof are hereinafter described with reference to the accompanying drawings. In Figs. 1 and 2, numeral 1 denotes a radome, numeral 5 denotes a lid, and numeral 10 denotes a foamy body. The radome 1 is molded as a dome shell of a specified size. The foamy body 10 has a curved surface which is identical with the inner surface of the radome 1 such that the foamy body 10 will be mounted in the radome 1 with no space in-between. The foamy body 10 is produced by the following process. In a water disperse system in an autoclave, an aliphatic hydrocarbon, for example, hexane, butane, pentane or the like, is impregnated into polymer particles of polystyrene. Further, a dielectric constant conditioner is added, and the polystyrene is preexpanded. By the preexpansion, polystyrene preexpanded beads are obtained. Next, the preexpanded beads are deposited in a mold and are fusion-molded. The inner surface of the mold is identical with the inner surface of the radome 1 such that the fusion-molded body 10 can be mounted in the radome 1 with no space in-between. The radome 1 and the lid 5 are made thin, out of the same polystyrene used for the foamy body 10 by a conventional method such as injection molding. The foamy body 10 is mounted in the radome 1, and the lid 5 is set at the opening of the radome 1. Thus, a dielectric lens is assembled. The radome 1 and the lid 5 are preferably bonded airtight by an adhesive or by ultrasonic welding. The radome 1 is desirably thin for a high antenna gain and is made to be 2mm, and more desirably 1mm or less, in thickness. In point of the antenna gain, it is further preferred that the space between the radome 1 and the foamy body 10 is not more than 0.5mm. If a pigment such as titanium oxide is added to the material of the radome 1, the radome 1 will absorb ultraviolet rays, which helps the foamy body 10 maintain its characteristic and lengthens the life of the lens. The followings are exemplary constituents of the foamy body 10. resin: polystyrene at a mixing ratio by weight of 100 dielectric constant conditioner: calcium titanate at a mixing ratio by weight of 60 foaming agent: butane The dielectric constant conditioner and the foaming agent were added to the resin, and the resin was preexpanded at an expansion ratio of 15. Then, the preexpanded resin was fusion-molded. As a result, a foamy body 10 which has a dielectric constant of 1.5 and has a weight of 250g was produced. The constituents of the materials of the radome 1, the lid 5 and the foamy body 10 are not limited to those described above. As the resin, polystyrene, polypropylene, polyethylene, etc. can be used. As the foaming agent, butane, pentane, etc. can be used. As the dielectric constant conditioner, calcium titanate, barium titanate, etc. can be used.

Claims: A method for producing a dielectric lens for an antenna, characterized by the following steps: molding a radome (1) as a dome shell of a specified size; forming pre-expanded beads with a specific dielectric constant; fusion-molding the pre-expanded beads into a dome (10) whose curved surface is substantially identical with an inner surface of the radome (1); mounting the fusion-molded dome (10) in the radome (1) and setting a lid (5) at an opening of the radome (1). A dielectric lens for an antenna formed by a method according to claim 1, comprising a radome (1) which is a dome shell of a specified size; pre-expanded beads shaped into a dome (10) whose curved surface is substantially identical with an inner surface of the radome (1). The dielectric lens for an antenna as claimed in claim 2, wherein the radome (1) contains an ultra-violet ray absorbing agent.

Inventors: YAMAMOTO KEIZO, YAMAMOTO, KEIZO, YAMAMOTO, KEIZO, C/O MURATA MANUFACTURING CO., LTD
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1994
Main classification: H01Q1
IPC codes: B29C67, H01Q1, B29D11, H01Q15, B29K105